Description

serhiy-storchaka

opened on Aug 16, 2026

Issue body actions

Bug report

The functions which read marshalled data from a FILE* never check ferror(), so a genuine I/O error is reported as if the file had simply ended.

>>> import _testcapi, os
>>> os.mkdir('d')
>>> _testcapi.pymarshal_read_object_from_file('d')
Traceback (most recent call last):
  ...
EOFError: EOF read where object expected

The real error is EISDIR. PyMarshal_ReadLongFromFile(), PyMarshal_ReadShortFromFile() and PyMarshal_ReadLastObjectFromFile() behave the same way.

Signals are not checked either, so a read interrupted by Ctrl-C is reported as EOFError instead of raising KeyboardInterrupt.

Doc/c-api/marshal.rst also documents only EOFError, ValueError and TypeError for these functions, which is inaccurate even today -- MemoryError is raised when the read buffer cannot be allocated.

Related: gh-101006 stopped EOFError from overriding an already set exception, but nothing sets OSError in the first place.

Linked PRs

• gh-155907: Raise OSError and KeyboardInterrupt when reading marshal data #155909
• gh-155907: Move PyMarshal C API tests to test_capi #157417
• gh-155907: Complete PyMarshal C API tests #157452

Activity

1. serhiy-storchaka

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   3.14bugs and security fixesbugs and security fixes

   3.15bugs and security fixesbugs and security fixes

   3.16new features, bugs and security fixesnew features, bugs and security fixes

   on Aug 16, 2026
2. serhiy-storchaka commented on Aug 29, 2026

   

   serhiy-storchaka

   on Aug 29, 2026

   MemberAuthor

   More actions

   The writing side has the same defect, so gh-155909 fixes it too.

   w_flush() and w_string() discard the result of fwrite(), so a failed or interrupted write to a FILE* is silently ignored:

   >>> import _testcapi, marshal
   >>> _testcapi.pymarshal_write_object_to_file(b'x' * 100000, '/dev/full', marshal.version)
   >>>

   PyMarshal_WriteObjectToFile() never converts the recorded WFERR_* code into an exception either, so an unmarshallable value is ignored as well, even though the function is documented to set the error indicator on failure. Only a lone TYPE_UNKNOWN byte reaches the file:

   >>> _testcapi.pymarshal_write_object_to_file(object(), 'f', marshal.version)
   >>> open('f', 'rb').read()
   b'\xbf'
3. serhiy-storchaka

   added a commit that references this issue on Sep 4, 2026

   gh-155907: Fix error handling in the marshal C API (GH-155909)

   aaae15c
4. serhiy-storchaka

   added

   type-featureA feature request or enhancementA feature request or enhancement

   topic-C-API

   and removed

   3.14bugs and security fixesbugs and security fixes

   3.15bugs and security fixesbugs and security fixes

   on Sep 4, 2026
5. serhiy-storchaka

   closed this as completedon Sep 4, 2026
6. clin1234

   added a commit that references this issue on Sep 12, 2026

   pythongh-155907: Fix error handling in the marshal C API (pythonGH-15…

   7ae6e67
7. vstinner

   added 3 commits that reference this issue on Sep 13, 2026

   gh-155907: Move PyMarshal C API tests to test_capi (#157417)

   bbdf31d

   pythongh-155907: Complete PyMarshal C API tests

   54028e2

   pythongh-155907: Complete PyMarshal C API tests

   a4945b5
8. johnslavik

   added a commit that references this issue on Sep 14, 2026

   pythongh-155907: Move PyMarshal C API tests to test_capi (python#157417)

   c262cdb
9. vstinner

   added a commit that references this issue on Sep 14, 2026

   gh-155907: Complete PyMarshal C API tests (#157452)

   7adb4cc
10. gpshead

    added 3 commits that reference this issue on Oct 11, 2026

    gh-159185: Document that PyMarshal_WriteObjectToFile can silently tru…

    b3714a5

    [3.15] gh-159185: Document that PyMarshal_WriteObjectToFile can silen…

    9ad8605

    [3.14] gh-159185: Document that PyMarshal_WriteObjectToFile can silen…

    f8a48fb

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